Re: [Qemu-devel] [RFC PATCH] implement vmware pvscsi device

[Stefan Hajnoczi <stefanha@gmail.com>]

On Fri, Apr 15, 2011 at 3:17 PM, Paolo Bonzini <pbonzini@redhat.com> wrote:
> On 04/15/2011 04:01 PM, Stefan Hajnoczi wrote:
>>
>> I think SCSI brings many benefits.  Guests can deal with it better
>> than these alien vdX virtio-blk devices, which makes migration easier.
>> It becomes possible to attach many disks without burning through free
>> PCI slots.  We don't need to update guests to add cache control,
>> discard, and other commands because they are part of SCSI.  We can
>> pass through more exotic devices.  The list goes on...
>
> And we also have to reimplement all of MMC. :)
>
> A few questions:
>
> 1) Do you have anything posted for the virtio-scsi spec?  I had started
> working on one, but I haven't yet made it final.  It included also
> hotplug/unplug.  I can send it out on Monday.

Nothing formal.  I'm trying to learn SCSI as I go along:

http://git.kernel.org/?p=linux/kernel/git/nab/lio-core-2.6.git;a=blob;f=include/linux/virtio_scsi.h;hb=refs/heads/tcm_vhost

That's the interface I'm using.  Requests are:

[Header][CDB][Data-out buffers*][Data-in buffers*][Footer]

The footer gets filled in with the response.

> 2) Have you thought about making scsi-disk and scsi-generic provide a
> logical unit rather than a target?  Otherwise passthrough of a whole host or
> target becomes hard or messy.

I haven't been working at the QEMU SCSI bus level.  I want to wire up
the Linux-iSCSI.org target stack straight to the guest.  This bypasses
the QEMU SCSI and block layers completely.

I agree that the BlockDriverState in QEMU is more of a LUN than a
target and passing through multiple block devices as LUNs should be
possible.  So we probably need to restructure as you suggested and/or
provide an indirection for LUN mapping.

Stefan